In physics, quintessence is a hypothetical form of dark energy, more precisely a scalar field, postulated as an explanation of the observation of an accelerating rate of expansion of the universe, rather than due to a true cosmological constant. The first example of this scenario was proposed by Ratra and Peebles (1988).[1] The concept was expanded to more general types of time-varying dark energy and the term "quintessence" was first introduced in a paper by Robert R. Caldwell, Rahul Dave and Paul Steinhardt.[2] It has been proposed by some physicists to be a fifth fundamental force[citation needed]. Quintessence differs from the cosmological constant explanation of dark energy in that it is dynamic; that is, it changes over time, unlike the cosmological constant which, by definition, does not change. It is suggested that quintessence can be either attractive or repulsive depending on the ratio of its kinetic and potential energy. Those working with this postulate believe that quintessence became repulsive about ten billion years ago, about 3.5 billion years after the Big Bang.[3]

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Quintessence is a scalar field with an equation of state where wq, the ratio of pressure pq and density ρ{\displaystyle \rho }q, is given by the potential energy V(Q){\displaystyle V(Q)} and a kinetic term:

Many models of quintessence have a tracker behavior, which according to Ratra and Peebles (1988) and Paul Steinhardtet al. (1999) partly solves the cosmological constant problem.[4] In these models, the quintessence field has a density which closely tracks (but is less than) the radiation density until matter-radiation equality, which triggers quintessence to start having characteristics similar to dark energy, eventually dominating the universe. This naturally sets the low scale of the dark energy.[5] When comparing the predicted expansion rate of the universe as given by the tracker solutions with cosmological data, a main feature of tracker solutions is that one needs four parameters to properly describe the behavior of their equation of state,[6][7] whereas it has been shown that at most a two-parameter model can optimally be constrained by mid-term future data (horizon 2015-2020).[8]

Some special cases of quintessence are phantom energy, in which wq < −1,[9] and k-essence (short for kinetic quintessence), which has a non-standard form of kinetic energy. If this type of energy were to exist, it would cause a big rip[10] in the universe due to the growing energy density of dark energy which would cause the expansion of the universe to increase at a faster-than-exponential rate.

Studies with quintessence dark energy found that it dominates gravitational collapse in a spacetime simulation, based on the holographic thermalization. These results show that the smaller the state parameter of quintessence is, the harder it is for the plasma to thermalize.[13]

In 2004, when scientists fitted the evolution of dark energy with the cosmological data, they found that the equation of state had possibly crossed the cosmological constant boundary (w = –1) from above to below. A proven no-go theorem indicates this situation, called the Quintom scenario, requires at least two degrees of freedom for dark energy models.[14]

The name comes from the classical elements in ancient Greece. The aether, a pure "fifth element" (quinta essentia in Latin), was thought to fill the universe beyond Earth. Similarly, modern quintessence would be the fifth known contribution to the overall mass-energy content of the universe. (The other four in the modern interpretation, different from the ancient ideas, are: baryonic matter; radiation – photons and the highly relativistic neutrinos, which may be considered hot dark matter; cold dark matter; and the term due to spatial curvature – loosely, gravitationalself-energy.)